Process for producing alkyl methacrylates with improved water and acid management

ABSTRACT

A process for producing alkyl methacrylates, in particular methyl methacrylate (MMA), includes production of methacrolein (MAL) in a first reaction stage; direct oxidative esterification (DOE) of the methacrolein with an alcohol, preferably methanol, to afford an alkyl methacrylate in a second reaction stage: and workup of the alkyl methacrylate crude product from the second reaction stage. An optimized workup of the reactor output from the oxidative esterification of methacrolein involves minimizing the amount of employed water, the amount of employed acid, and/or the amount of aqueous waste streams, through an optimized recycling of the generated process water streams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/754,076, filed on Mar. 23, 2022, which is a National Stage entryunder § 371 of International Application No. PCT/EP2020/075679, filed onSep. 15, 2020, and which claims the benefit of European Application No.19199483.9, filed on Sep. 25, 2019. The contents of each of theseapplications is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a process for producing alkylmethacrylates, in particular methyl methacrylate (MMA), comprisingproduction of methacrolein (MAL) in a first reaction stage, directoxidative esterification (DOE) of the methacrolein with an alcohol,preferably methanol, to afford an alkyl methacrylate in a secondreaction stage and workup of the alkyl methacrylate crude product fromthe second reaction stage.

The present invention especially relates to an optimized workup of thereactor output from the oxidative esterification of methacrolein,wherein the amount of employed water, the amount of employed acid and/orthe amount of aqueous waste streams are minimized through an optimizedrecycling of the generated process water streams.

PRIOR ART

Methyl methacrylate is employed in large amounts to produce polymers andcopolymers with other polymerizable compounds. Methyl methacrylate (MMA)is also an important building block for a very wide variety of specialesters based on methacrylic acid (MAA) which are producible byesterification of the acid/transesterification of MMA with thecorresponding alcohol. There is therefore a great interest in verysimple, economic and environmentally friendly production processes forthis starting material.

Methyl methacrylate (MMA) is today produced by a very wide variety ofprocesses proceeding from C₂—, C₃- oder C₄-building blocks. In one ofthese processes MMA is obtained by oxidation of isobutylene ortert-butanol with atmospheric oxygen in the gas phase over aheterogeneous contact to afford methacrolein (MAL) and subsequentoxidative esterification reaction of methacrolein using methanol. Thisprocess, developed by ASAHI, is described inter alia in documents U.S.Pat. Nos. 5,969,178 and 7,012,039. The very high energy requirements area particular disadvantage of this process.

The production of MMA according to the so-called Asahi process iscarried out starting from C4 (isobutene or tert-butanol) withintermediate isolation of MAL and subsequent direct oxidativeesterification (“DOE” for short) of the MAL with methanol to afford MMAand forming methacrylic acid (MAA) as a byproduct by reaction withwater.

In a development of the process the methacrolein is obtained frompropanal and formalin in the first stage. Such a process is described inWO 2014/170223.

The following scheme 1 shows the reaction matrix for the production ofMMA, the production of methacrolein starting from ethylene, synthesisgas and formaldehyde being described by way of example. As explainedhereinabove the methacrolein is also obtainable starting from isobuteneor tert-butanol.

In the oxidative esterification (DOE reaction) methacrylic acid isformed in the presence of water which upon continuous performance of thereaction is typically present in the oxidative esterification reactor ina stationary concentration between 2% and 20% by weight. If the DOEreaction is performed at constant pH the resulting methacrylic acid isat least partially neutralized with alkaline or basic auxiliaries, inthe simplest case with alkali compounds.

The crude product from the oxidative esterification typically containsthe alkyl methacrylate as the main reaction product, unconvertedreactants, in particular methacrolein and alcohol, small amounts ofwater and various byproducts, such as methacrylic acid (MAA) and saltsthereof (for example sodium methacrylate), acetals (for exampledimethoxyisobutene DMIB) and also further high-boiling components, forexample addition products and Diels-Alder products, for examplehydroxyisobutyric acid and corresponding esters thereof, dimericmethacrolein (DIMAL) and esters thereof (DIMAL esters). The high-boilingbyproducts must often be removed to obtain the desired alkylmethacrylate product (for example MMA) in a suitable purity forpolymerization, typically of markedly greater than 99% by weight.

U.S. Pat. No. 5,969,178 and the prior art referred to therein describesprocess variants for oxidative conversion of isobutene or tert-butanolinto methacrolein and subsequent oxidative esterification to afford MMA.In a first distillation stage a mixture of methacrolein and methanol iseremoved from the crude product of the oxidative esterification below thetop of the column while low-boiling constituents are removed overhead.The MMA-containing bottoms are subsequently passed, together with anunsaturated hydrocarbon, into a second distillation stage in which anazeotrope of methanol and saturated hydrocarbons is removed overhead.The bottoms containing the crude MMA are sent to a further workup whilemethanol is isolated from the fraction obtained overhead using a phaseseparator and a third distillation column and recycled into the reactor.It must be taken into account that due to the azeotrope formed themethanol may contain a relatively large amount of water and thus needsto be sent for dewatering.

As an alternative to this process U.S. Pat. No. 5,969,178 disclosesworkup in only one column wherein the feed must necessarily be locatedabove the column bottom in said column. Low-boiling constituents areremoved from the reactor output overhead in this column. A mixture ofcrude MMA and water which must be sent for further workup remains in thecolumn bottom. A mixture of methacrolein and methanol for recycling intothe reactor is finally removed from the column via a sidestream. U.S.Pat. No. 5,969,178 indicates that such a process is difficult to performon account of a wide variety of azeotropes. U.S. Pat. No. 5,969,178offers no solution for the removal of MMA from methacrylatesalt-containing aqueous solutions which are necessarily generated in thedescribed process mode.

U.S. Pat. No. 7,012,039 discloses a slightly different workup of thereactor output from the oxidative esterification. The use of alead-containing catalyst which obviously also emits lead into thereaction solution renders subsequent workup in particular more complexsince insoluble lead salts are formed in the process and require specialremoval at great cost and complexity. In a first distillation stagemethacrolein is distilled off overhead via sieve trays and the aqueous,MMA-containing mixture is passed from the column bottom into a phaseseparator. In said separator the mixture is adjusted to a pH of about 2by addition of sulfuric acid. This is followed by separation of thesulfuric acid-containing water from the organic/oil phase bycentrifugation. In a further distillation this organic phase isseparated into high-boiling constituents and an MMA-containing phasewhich is withdrawn overhead. The MMA-containing phase is subsequentlyseparated from low-boiling constituents in a third distillation. This isfollowed by a fourth distillation for final purification.

The problem with the processes according to U.S. Pat. No. 7,012,039 isthe sulfuric acid which must be added in large amounts and can have acorrosive effect in parts of the plant. These parts, such as especiallythe phase separator or else the second distillation column, musttherefore be manufactured from materials suitable therefor. Norecirculation of acid-containing aqueous streams apparent to a personskilled in the art is derivable from this disclosure. The processaccording to U.S. Pat. No. 7,012,039 further provides no option forrecovery of methacrylic acid or the residual methanol remaining in theproduct.

WO 2014/170223 describes a similar process to U.S. Pat. No. 7,012,039.It differs in that the pH in the oxidative esterification is adjusted byaddition of a methanolic sodium hydroxide solution to a recirculatingsystem. The pH regulation serves, inter alia, to protect the catalyst.Furthermore, removal of the aqueous phase in the phase separation issimpler on account of the salt content. However this also has the resultthat the methacrylic acid formed is present as the sodium salt and islater removed and discarded with the aqueous phase. In the variantcomprising sulfuric acid addition in the phase separation the free acidis recovered but at the cost of generating sodium (hydrogen)sulfatewhich can lead to other problems upon disposal.

WO 2017/046110 describes a process for producing MMA, wherein the crudeproduct from the oxidative esterification is worked up by distillationof the aqueous phase from an extraction which contains methanol, atleast one alkali metal salt, methacrylic acid and a strong inorganicacid. The low-boiling fraction from this distillation stage, whichcontains mainly methanol, is recycled into the oxidative esterification.The aqueous bottoms fraction is discharged and disposed of. A sidestreamfrom the distillation containing water and methacrylic acid may be sentto the extraction. Addition of a strong acid during the extraction ofthe crude product neutralizes salts of methacrylic acid, thus allowingthe free methacrylic acid to be obtained conveniently.

WO 2019/042807 describes a process for producing PMMA resins whichcomprises initially producing the monomer. After the oxidativeesterification in a reactor the MMA crude product is subjected toaftertreatment by addition of an organic and/or mineral acid to effecthydrolytic cleavage of the byproduct dimethoxyisobutene.

The disadvantage of the prior art processes for workup of the crudeproduct from the oxidative esterification is that relatively largeamounts of acid and water are supplied to the process at various points,thus also generating a relatively large proportion of aqueous wastestreams which are contaminated with acid and/or organic components andmust be disposed of. The addition of strong acids also necessitateselevated equipment cost and complexity in the corresponding processpart, such as especially implementation in corrosion-resistantmaterials.

Problem

Having regard to the prior art the problem addressed by the presentinvention is accordingly that of providing a technically improvedprocess for oxidative esterification of methacrolein which is not besetwith the above-described disadvantages of conventional processes. Theproblem addressed by the present invention is especially that ofproviding an improved workup of the crude product from the oxidativeesterification of methacrolein, wherein the amount of employed water andof employed acid and also the amount of aqueous waste streams can bereduced. The problem addressed is especially that of providing a processoperable with the lowest possible disposal cost and complexity, inparticular through reduced generation of aqueous and organicconstituents and acids in the waste stream.

The process shall also be cost-effective compared to the prior art, forexample in respect of the materials to be used in the construction ofthe plant. The process should also make it possible to achieve thegreatest possible yield of alkyl methacrylate and to recover methacrylicbyproducts, such as methacrylic acid. The process should in principlemake it possible to convert and recycle the greatest possible proportionof byproducts.

Solution to the Problem

It has surprisingly been found that the addition of acid and water andthe amount of aqueous wastewater can be minimized through an optimizedmanagement of the generated process water streams. It has especiallybeen found that optimized and targeted distribution of the aqueousbottoms from the distillation stage for recovery of the alcohol canachieve savings in employed water and/or employed acid and a reductionin the wastewater stream requiring withdrawal from the process.

The above-described problems are solved according to the invention by aprocess for producing alkyl methacrylates, wherein in a first reactionstage in a reactor I methacrolein (MAL) is produced and in a secondreaction stage in a reactor II this is oxidatively esterified with analcohol, preferably methanol, in the presence of an oxygen-containinggas and with formation of reaction water in the liquid phase to affordan alkyl methacrylate, preferably to afford methyl methacrylate (MMA),characterized in that

-   -   a. the workup of the reaction mixture from reactor II to afford        alkyl methacrylate comprises at least one distillation and at        least one extraction;    -   b. an aqueous phase containing alcohol and an alkali metal        and/or alkaline earth metal salt of a Brønsted acid from the        extraction is treated in at least one distillation in a column        II (distillation stage II) in such a way that in the bottom of        the column II a process water stream (bottoms fraction W4)        containing reaction water and alkali metal and/or alkaline earth        metal salt of a Brønsted acid is formed, wherein in this process        water stream the content of alcohol and alkyl methacrylate is        less than 5% by weight based on the total process water stream,        and    -   c. this process water stream from the bottom of the column II        (bottoms fraction W4) is partially discharged from the process        and sent for disposal and partially recycled into the workup of        the reaction mixture from reactor II.

DESCRIPTION OF THE INVENTION

In the context of the invention the term stream, phase or fractioncontaining a reactant, product and/or byproduct of the claimed processis to be understood as meaning that the recited compounds are to befound in the respective stream, for example the predominant proportionof the reactant, product and/or byproduct may be found in thecorresponding stream. Further constituents may in principle be presentin addition to the recited compounds. The naming of the constituentsoften serves to elucidate the respective process step.

It is preferable when the at least one distillation of the processaccording to the invention, for example the distillation stages I; II,III, and IV described hereinbelow, is performed in distillation columns.The distillation columns are, corresponding to the distillation stage,referred to as I; II, III, IV etc. Typical embodiments of distillationcolumns are known to those skilled in the art. It is typically possibleto employ tray columns, fitted for example with sieve trays, cascadetrays, turbogrid trays and/or louvred trays, or packed columns, forexample random-packed columns (for example with Raschig Super-Rings fromRaschig) or columns having regular packings (for example Mellapak fromSulzer). The distillation stages typically afford at least onehigh-boiling bottoms fraction and at least one low-boiling topsfraction. The distillation temperature in the different distillationstages is chosen by those skilled in the art according to thedistillation pressure, the composition of the mixture to be separated,the number of trays and configuration of the distillation column andalso further factors. The distillation temperature is preferably in therange from 20° C. to 120° C.

It is preferable when the alcohol is methanol and the alkyl methacrylateis methyl methacrylate (MMA).

It is preferable when the Brønsted acid (hereinbelow also referred to asacid S) is a strong acid, in particular an acid having a pK_(a) lowerthan the pK_(a) of methacrylic acid. The Brønsted acid preferably has apK_(a) of less than 3, particularly preferably less than 2. It ispossible to employ strong inorganic acids, such as sulfuric acid orphosphoric acid, or strong organic acids, such as methanesulfonic acidor toluenesulfonic acid. The Brønsted acid is preferably sulfuric acid.

The process according to the invention is preferably operated incontinuous or semi-continuous fashion. With the exception ofdiscontinuous discharging of waste streams the process is preferablyoperated in continuous fashion.

It is preferable when the process water stream from the bottom of thecolumn II is partially recycled into the extraction, wherein thisrecycled process water stream is contacted with an alkyl methacrylate-and alcohol-containing organic phase (in particular organic phase P1) inthe extraction.

It is preferable when the process water stream from the bottom of thecolumn II is the bottoms fraction W4 from the preferred processdescribed hereinbelow.

It is preferable when the process water stream from the bottom of thecolumn II is partially recycled into a reactor III (in particular acetalcleaver III wherein acetals present in stream W1 are cleaved), whereinthis recycled process water stream is contacted in reactor III with analkyl methacrylate- and alcohol-containing organic phase (in particularbottoms fraction W1) which has a content of methacrolein acetal (inparticular of dimethoxyisobutene) of less than 3% by weight based on theorganic phase.

The extraction is preferably performed in an extraction column and/or ina serially arranged series of at least two mixer-settler apparatuses andthe process water stream from the bottom of the column II is added belowthe top of the extraction column or in the mixer region of amixer-settler apparatus and water, in particular demineralized water, isoptionally added in the top region of the extraction column.

The water supplied to the process according to the invention is inparticular demineralized water.

It is preferable when the addition of water and optionally of a Brønstedacid to the extraction is carried out above the addition of the processwater stream from the bottom of the column II.

The addition of a Brønsted acid is preferably carried out in reactor IIIand this addition is chosen such that in continuous operation a pH inthe range from 1.5 to 2.5 is established in the process water streamfrom the bottom of the column II (in particular the bottoms fractionW4).

In a preferred embodiment the addition of a Brønsted acid into theprocess according to the invention is carried out exclusively via theaddition in reactor III. It is also conceivable to perform the acidaddition at different points in the process in order to allow moreprecise adjustment of the different pH values for the different processoperations.

The extraction preferably affords an organic phase containing alkylmethacrylate and methacrylic acid and the organic phase from theextraction (in particular the organic phase P1 or the tops fraction W5from distillation stage III) is separated in a distillation stage IV(column IV) into a bottoms fraction containing alkyl methacrylate (inparticular bottoms fraction W8) and a lower-boiling tops fraction (inparticular tops fraction W7).

The fraction from the column IV is preferably admixed with water andsubsequently separated in a phase separator I into an organic phase (inparticular organic phase P3) and into an aqueous phase (in particularaqueous phase P4).

It is preferable when the aqueous phase from the phase separator I (inparticular aqueous phase P4) is mixed with at least one Brønsted acid ina reactor IV, wherein ester byproducts present in the aqueous phase fromthe phase separator I are cleaved and alcohol is recovered and whereinthe process water stream from the bottom of the column II is optionallypartially passed into the reactor IV.

It is preferable when the product from the reactor IV is completely orpartially passed into the column II.

In an alternative embodiment the acetal hydrolysis (DMIB+water→MAL toMeOH) in reactor III and the partial ester hydrolysis carried out inreactor IV may be combined in one reaction vessel.

It is preferable when the aqueous phase from phase separator I iscompletely or partially passed into the column II.

It is preferable when the reaction mixture from reactor II is separatedin a distillation stage I (column I), wherein methacrolein and in partalcohol are removed via the tops fraction and recycled to the reactorII.

It is preferable when methacrolein is completely or partially added tothe distillation stage I and passed to the reactor II via the topsfraction from the distillation stage I.

It is preferable when the alcohol is methanol and the alkyl methacrylateis methyl methacrylate. The Brønsted acid is preferably sulfuric acid.

First Reaction Stage (MAL Production)

In a first reaction stage the process according to the inventioncomprises the production of methacrolein (MAL) in a reactor I. Accordingto the invention the first stage of the process for synthesis of themethacrolein is freely choosable. The first reaction stage of theprocess may comprise either a first stage synthesis based ontert-butanol or isobutylene or a first stage synthesis based on propanaland formalin.

In the case of production of methacrolein (MAL) based on propanal andformalin there are in principle two suitable process variants whichprovide methacrolein in a quality that may be employed in the secondreaction stage (direct oxidative esterification, DOE reaction). Propanaland formalin may be reacted in a stirred reactor or pumped-circulationreactor at temperatures of 20° C. to 120° C. at pressures of 1 bar to 10bar. This typically requires reaction times of more than 10 min toachieve sufficient conversions. Propanal and formalin may also bereacted to afford MAL at an average pressure between 10 and 100 bar andat relatively high temperatures between 120° C. and 250° C., thereaction achieving the desired high yields with a reaction time of 2seconds to 20 seconds.

Processes for producing methacrolein are known to those skilled in theart and described for example in Ullmanns Encyclopedia of IndustrialChemistry, 2012, Wiley-VCH Verlag GmbH, Weinheim (DOI:10.1002/14356007.a01_149.pub2).

The first reaction stage may also be a process proceeding from C4 rawmaterials, especially oxidation of tert-butanol and/or isobutene.Isobutene or tert-butanol is typically subjected to gas-phase reactionover a heterogeneous contact with oxygen-containing gases and preferablywith steam at temperatures of above 300° C. A multiplicity ofsubvariants and employable catalyst systems and also insulation optionsare described in the relevant prior art. An overview thereof may befound for example in “Trends and Future of Monomer-MMA Technologies”, K.Nagai & T. Ui, Sumitomo Chemical Co., Ltd., Basic Chemicals ResearchLaboratory, 2005(https://www.sumitomo-chem.co.jp/english/rd/report/theses/docs/20040200_30a.pdf).

It is preferable when the first reaction stage in reactor I is areaction of propanal with formalin. The first reaction stage mayoptionally comprise a distillation column for removal of low boilers,such as remaining propanal, and/or of high boilers (for example dimericmethacrolein).

Second Reaction Stage (Direct Oxidative Esterification DOE)

In a second reaction stage the process according to the inventioncomprises the oxidative esterification (DOE reaction) of themethacrolein with an alcohol, preferably methanol, in a reactor II, toobtain an alkyl methacrylate, preferably methyl methacrylate (MMA).

The direct oxidative esterification (DOE reaction) is preferablyperformed in the liquid phase at a pressure of 2 to 100 bar, preferablyat a pressure in the range from 2 to 50 bar, and a temperature in therange from 10° C. to 200° C. over a heterogeneous catalyst. Theheterogeneous catalyst is generally selected from supported,gold-containing nanoparticles having a particle size of less than 20 nm,preferably in the range from 0.2 to 20 nm.

The direct oxidative esterification is typically performed with pHcontrol to ensure optimal activity of the catalyst; the pH is preferablycontrolled to pH 6-8, particularly preferably to pH 7.

The second reaction stage preferably comprises reaction of methacroleinand an alcohol in the presence of an oxygen-containing gas at moderatetemperatures between 20° C. and 150° C. at moderate pressures between 1and 20 bar in the presence of a heterogeneous particulate noblemetal-containing catalyst preferably having a particle size of 1 to 300μm, wherein larger particles are also employable.

The prior art describes a multiplicity of catalysts for this oxidativeesterification of MAL with methanol to afford MMA. U.S. Pat. No.6,040,472 describes Pd—Pb-containing catalysts on an oxidic support; EP1 393 800 describes gold-containing catalysts comprising gold particlesdistributed on a silicon oxide or TiO₂/SiO₂ support; EP 2 177 267 and EP2 210 664 describe nickel-containing catalysts having a shell structure;EP 2 210 664 discloses a catalyst comprising nickel oxide and goldnanoparticles on a support; WO 2017/084969 describes catalyst systemsbased on two or more mixed oxides as the support which likewise comprisenanoparticulate gold as the active component in addition to cobaltoxide.

The reactor II typically comprises a feed of methacrolein (MAL), a feedof alcohol (in particular methanol), a feed of oxygen and/or air and afeed of a base.

The reactants or a portion of the reactants, in particular base andMeOH, may optionally be mixed in an optional mixer before introductioninto the reactor II.

Processes for direct oxidative esterification of methacrolein are knownto those skilled in the art. Further details concerning the secondreaction stage are described for example in U.S. Pat. Nos. 5,969,178,7,012,039, WO 2014/170223 und WO 2019/042807.

In a preferred embodiment the workup of the reaction mixture fromreactor II comprises the steps of:

-   -   i. separating the crude product of the oxidative esterification        from reactor II in a distillation stage 1, wherein methacrolein        and in part alcohol are removed in the tops fraction and        recycled to the reactor II and wherein a bottoms fraction W1        containing alkyl methacrylate, methacrylic acid and/or salts        thereof, alcohol and water is obtained;    -   ii. optionally introducing the bottoms fraction W1 into a        reactor III and adding at least one acid S, wherein acetals        present in stream W1 are cleaved and wherein a stream W2 is        obtained;    -   iii. extracting the stream W1 or W2 with water and separating        into an organic phase P1 containing alkyl methacrylate and        methacrylic acid and an aqueous phase P2 containing water, acid        S and/or salts thereof, alcohol, methacrylic acid and/or salts        thereof;    -   iv. separating the aqueous phase P2 from the extraction in a        distillation stage II to obtain a tops fraction W3 containing        mainly alcohol and a bottoms fraction W4 containing water, acid        S and/or salts thereof and methacrylic acid and/or salts        thereof;    -   v. separating the organic phase P1 from the extraction in a        distillation stage III to obtain a tops fraction W5 containing        alkyl methacrylate and a bottoms fraction W6 containing        methacrylic acid;    -   vi. separating the tops fraction W5 from distillation stage III        in a distillation stage IV to obtain a tops fraction W7 and a        bottoms fraction W8 containing alkyl methacrylate;    -   vii. mixing the tops fraction W7 from the distillation stage IV        with water and separating into an organic phase P3 and an        aqueous phase P4 in a phase separator I;    -   viii. optionally mixing the aqueous phase P4 from phase        separator I with at least one acid S in reactor IV, wherein        ester byproducts present in P4 are cleaved and alcohol is        recovered and wherein an aqueous phase P5 is obtained;    -   wherein the bottoms fraction W4 from distillation stage II is        completely or partially passed into one or more of the process        parts selected from reactor III, extraction, phase separator I        and reactor IV.

Typically the aqueous phases, for example P2 and P4, are the heavierphase and the organic phases, for example P1 and P3, the lighter phaseof the respective separation.

The preferred steps of workup of the reaction mixture from reactor IIare more particularly described hereinbelow.

Distillation Stage I (Step i)

The process according to the invention preferably comprises separatingthe reaction mixture from the oxidative esterification from reactor IIin a distillation stage I, in particular in a distillation column I,wherein methacrolein and in part alcohol are removed and recycled to thereactor II and wherein a stream W1 containing alkyl methacrylate,methacrylic acid and/or salts thereof, alcohol and water is obtained.

In a preferred embodiment of the process according to the invention themethacrolein is not or only partially passed directly into the reactorII but rather introduced into the process via the distillation stage I.It is preferable when methacrolein is completely or partially added tothe distillation stage I and passed to the reactor II via thelow-boiling tops fraction from the distillation stage I.

Reactor III—Acetal Cleaver (Optional Step ii)

The process according to the invention preferably comprises introducingthe bottoms fraction W1 from distillation stage I into a reactor III(acetal cleaver) and adding at least one acid S, wherein acetals presentin the stream W1 are cleaved and wherein a stream W2 is obtained.

In reactor III the acetal byproduct dimethoxyisobutene (DMIB) ispreferably cleaved into methacrolein (MAL) and methanol (MeOH). Acetalcleavage is described for example in WO 2019/042807 and JP 11-302224A.Removal of the acetal byproducts in the optional process step ii) canespecially reduce discoloration in descendent products of the alkylmethacrylate, such as polymers and moulding materials.

In addition, salts of methacrylic acid, such as especially alkali metalmethacrylates, are typically neutralized to form free methacrylic acidin reactor III.

A mixing of the bottoms fraction W1 with at least one acid S andoptionally water is typically carried out in the reactor III. ReactorIII may be configured in a manner known to those skilled in the art, forexample as a tubular reactor preferably fitted with a static mixer, as astirred reactor or as a combination thereof.

Reactor III preferably comprises an acid feed I, wherein at least oneacid S, preferably sulfuric acid, is supplied. The at least one acid Smay also be supplied (exclusively or partially) via the recycling of thebottoms fraction W4 from the distillation stage II. Reactor III mayoptionally comprise a water feed in addition to the acid feed I.

The pH in the reactor III is preferably in the range from 0 to 7.0,preferably 0.5 to 5.0.

The addition of the at least one acid S (preferably sulfuric acid) intothe reactor III is typically chosen such that the aqueous phase P2 inthe extraction has a pH in the range of not more than pH 4, preferablypH 1.5 to pH 3.

In a preferred embodiment the optional process step ii) is realized andthe addition of acid S (preferably sulfuric acid) in reactor III,especially the addition of the acid S via the acid feed I, is chosensuch that in continuous operation a pH in the range from 0 to 3,preferably of 2, is established in the bottoms fraction W4 fromdistillation stage II.

In a preferred embodiment the optional process step ii) is realized andthe addition of fresh acid S (preferably sulfuric acid) into the processaccording to the invention is carried out exclusively via the feed intoreactor III, preferably via the acid feed I.

Extraction (Step iii)

The process according to the invention comprises at least oneextraction.

The process preferably comprises extraction of the stream W1 (withoutoptional step ii)) or W2 (with optional step ii)) with water andseparation into an organic phase P1 (typically the light phase)containing alkyl methacrylate and methacrylic acid and an aqueous phaseP2 (typically the heavy phase) containing water, acid S and/or saltsthereof, alcohol, methacrylic acid and/or salts thereof. The addition ofwater is preferably carried out in the upper part of the column.

The organic phase P1 typically contains mainly the alkyl methacrylateand organic byproducts of the reaction, such as methacrylic acid andfurther high-boiling byproducts, and smaller amounts of water andmethanol. The aqueous phase P2 typically contains little in the way oforganic products and contains mainly water and methanol and alkali metalor alkaline earth metal salts from the neutralization and salts ofmethacrylic acid, in particular alkali metal salts of methacrylic acid.

It is preferable when at least one acid S is added during theextraction. The addition of the at least one acid S (preferably sulfuricacid) in the extraction is preferably chosen such that the aqueous phaseP2 in the extraction has a pH in the range of not less than 3. Theaddition of the acid S in the extraction typically neutralizes salts ofmethacrylic acid, such as alkali metal methacrylates in particular.

Water may optionally be added to the extraction via an optional feed II.Acid S may optionally be added to the extraction, for example via anoptional acid feed and/or together with the water feed II. The water andoptionally the acid S may also be supplied to the extraction(exclusively or partially) via the recycling of the bottoms fraction W4from the distillation stage II.

The extraction in step iii) is preferably performed in an extractioncolumn. Typical embodiments of extraction columns are known to thoseskilled in the art. Typically employable columns include tray columns orpacked columns, for example random-packed columns (for example withRaschig rings) or columns comprising regular packings (for exampleMellapak from Sulzer).

In a preferred embodiment the addition of water to the extraction iscarried out via the water feed II in the upper region of a distillationcolumn, preferably to the first extraction stage, preferably above anyrecycling of the bottoms fraction W4 from the distillation stage II.

In a preferred embodiment the extraction (step iii) is performed in anextraction column and the bottoms fraction W4 from distillation stage IIis added in the middle region of the extraction column, preferably belowthe first extraction stage, particularly preferably below the secondextraction stage. It is further preferable when the addition of waterand optionally acid S to the extraction in the extraction column iscarried out above the addition of the bottoms fraction W4 fromdistillation stage II.

The organic phase P1 from the extraction, which contains the greatestproportion of the desired alkyl methacrylate product, is preferablyfurther purified via the distillation stages III and IV and optionally Vand VI described hereinbelow. The organic phase P1 which contains thepredominant proportion of the alkyl methacrylate is freed initially ofhigher-boiling components (distillation stage III) and subsequently oflower-boiling components (distillation stage IV).

The aqueous phase W4 typically contains mineral salts of thecorresponding Brønsted acid, wherein the salt content is in the rangefrom 0.5% to 15% by weight based on W4. The recycling of thesalt-containing phase W4 into the extraction typically brings about animprovement in extraction performance. In general this effect is basedon a density which is elevated compared to DM water and thus a higherdensity difference compared to the organic phase.

Distillation Stage II—Alcohol Recovery (Step iv)

The process according to the invention preferably comprises separatingthe aqueous phase P2 from the extraction in a distillation stage II(preferably in a distillation column II) to obtain a low-boiling topsfraction W3 containing mainly alcohol and a high-boiling bottomsfraction W4 containing water, acid S and/or salts thereof andmethacrylic acid and/or salts thereof.

It is preferable when the tops fraction W3 is completely or partiallyrecycled into the reactor II.

According to the invention the aqueous bottoms fraction from thedistillation stage II is recycled to one or more different points of theprocess according to the invention, thus saving water and/or acid S andreducing the amount of aqueous waste streams. According to the inventionthe bottoms fraction W4 from distillation stage II is completely orpartially passed into one or more of the process parts selected fromreactor III, extraction, phase separator I and reactor IV.

In a preferred embodiment of the process a portion of the bottomsfraction W4 from distillation stage II is continuously and/ordiscontinuously discharged from the process (as a purge).

The bottoms fraction W4 from distillation stage II is preferablycompletely or partially recycled into the extraction (step iii).Preferred embodiments of the addition of W4 to the extraction aredescribed hereinabove.

The bottoms fraction W4 from distillation stage II is preferablycompletely or partially passed into the phase separator I (step vii).

In a preferred embodiment the optional process step ii) (acetal cleaver)is realized and the bottoms fraction W4 from distillation stage II iscompletely or partially recycled into the reactor III (acetal cleaver).

In a preferred embodiment the optional process step viii) is realizedand the bottoms fraction W4 from distillation stage II is completely orpartially passed into the reactor IV (ester hydrolysis).

In a preferred embodiment the recycling of the bottoms fraction W4 iscarried out into two or more of the recited process parts.

The aqueous bottoms fraction W4 preferably has a pH in the range from 1to 3, preferably 1.5 to 2.5. The aqueous bottoms fraction W4 preferablycontains more than 60% by weight, preferably more than 80% by weight,based on the total weight of W4, of water. The bottoms fraction W4preferably contains less than 10% by weight, preferably less than 5% byweight, particularly preferably less than 1% by weight, based on thetotal weight of W4, of alcohol, in particular methanol.

Distillation Stage III—Removal of the High Boilers (Step v)

The process according to the invention preferably comprises theseparating of the organic phase P1 from the extraction in a distillationstage III (in particular in a distillation column III) to obtain alow-boiling tops fraction W5 containing alkyl methacrylate and ahigh-boiling bottoms fraction W6 containing methacrylic acid.

In one embodiment a portion of the tops fraction W5 may be passed into aphase separator II. The aqueous phase may be supplied to reactor IV.

In a preferred embodiment the bottoms fraction W6 containing methacrylicacid may be passed into an optional distillation stage VI (distillationcolumn VI), wherein the amount of alkyl methacrylate in W6 is reducedand the tops fraction from the distillation stage VI containing alkylmethacrylate may be recycled into the distillation stage III.

In a preferred embodiment methacrylic acid may be obtained as a furtherproduct from the bottoms fraction W6 and/or from the bottoms fractionfrom the optional distillation stage VI. Details of this are describedfor example in WO 2017/046110.

Alternatively, one or more or all distillation stages III, IV and V forpurification of the alkyl methacrylate may also be replaced by acrystallization.

Distillation Stage IV—Removal of the Low Boilers (Step vi)

The process according to the invention preferably comprises theseparating of low-boiling tops fraction W5 from distillation stage IIIin a distillation stage IV (in particular in a distillation column IV)to obtain a low-boiling tops fraction W7 and a high-boiling bottomsfraction W8 containing alkyl methacrylate.

The bottoms fraction W8 may preferably be passed into a further optionaldistillation stage V (in particular distillation column V), wherein afinal purification of the alkyl methacrylate is carried out and thealkyl methacrylate as tops fraction from the optional distillation stageV is discharged from the process as product.

The bottoms fraction from the optional distillation stage V mayoptionally be passed into the distillation stage III and/or thedistillation stage VI.

Phase Separator I (Step vii)

The process according to the invention preferably comprises the mixingof the tops fraction W7 from the distillation stage IV with water andseparation into an organic phase P3 (typically the light phase) and anaqueous phase P4 (typically the heavy phase) in a phase separator I.

Typical configurations of phase separators are known to those skilled inthe art.

From the phase separator I the organic phase P3 may preferably bedischarged from the process as an organic waste stream completely orpartially, continuously or discontinuously. It is preferable when theorganic phase P3 from the phase separator I is completely or partiallypassed into an optional distillation stage VII (MAL recovery) in whichmethacrolein is recovered and sent to the oxidative esterification inreactor II.

In an alternative embodiment the aqueous phase P4 from the phaseseparator I is completely or partially passed into the distillationcolumn II (alcohol recovery). This is carried out in particular if theoptional process step viii) has not been realized.

It is also possible to discharge the aqueous phase P4 from phaseseparator I as an aqueous wastewater stream completely or partially,continuously and/or discontinuously.

The phase separator I preferably comprises a water feed III. The watermay also be supplied (exclusively or partially) via the recycling of thebottoms fraction W4 from the distillation stage II.

Reactor IV—Ester Hydrolysis (Optional Step Viii)

The process according to the invention preferably comprises mixing theaqueous phase P4 from phase separator I with at least one acid S inreactor IV (ester hydrolysis), wherein ester byproducts present in P4are cleaved and alcohol is recovered and wherein an aqueous phase P5 isobtained.

In optional reactor IV ester byproducts which have a lower boiling pointthan methyl methacrylate, in particular saturated esters such as alkylisobutyrates and alkyl propionates, for example methyl isobutyrate,methyl propionate, are typically hydrolyzed, thus allowing alcohol,preferably methanol, to be recovered.

In a preferred embodiment of the process the optional process step viii)is realized and the aqueous phase P5 from reactor IV (ester hydrolysis)is completely or partially passed into the distillation column II. Thealcohol recovered in the ester hydrolysis is typically supplied to thereactor II via the tops fraction W3.

A portion of the aqueous phase P5 from reactor IV may also becontinuously and/or discontinuously discharged as an aqueous wastewaterstream.

Reactor IV preferably comprises an acid feed IV, wherein at least oneacid S, preferably sulfuric acid, is supplied. The at least one acid Smay also be supplied (exclusively or partially) via the recycling of thebottoms fraction W4 from the distillation stage II. Reactor IV mayoptionally comprise a water feed in addition to the acid feed IV.

DESCRIPTION OF THE FIGURE AND LIST OF REFERENCE NUMERALS

FIG. 1 shows by way of example a possible schematic flow diagram of thesecond process step (oxidative esterification of MAL and workup of theproduct stream) of the process according to the invention for producingalkyl methacrylate. Reactor I of the process according to the inventionfor MAL synthesis is not shown.

-   -   (1) MAL feed into reactor II    -   (2) Reactor II for oxidative esterification of MAL    -   (3) Alcohol (in particular methanol) feed into reactor II    -   (4) Oxygen and/or air feed into reactor II    -   (5) Base feed into reactor II    -   (6) Distillation column I for removal of MAL    -   (7) Low-boiling fraction containing MAL and alcohol for        recycling into reactor II (recycling stream)    -   (8) Reactor III (acetal cleaver) for cleavage of acetal        byproducts (for example dimethoxyisobutene (DMIB) to afford MAL        and MeOH) (optional)    -   (9) Extraction    -   (10) Optional acid feed I to (8)    -   (11) Optional water feed II to (9)    -   (12) Distillation column II for recovery of alcohol    -   (13) Low-boiling fraction from distillation column II containing        alcohol for recycling into reactor II    -   (14) Bottoms fraction from distillation column II containing        water, acid and alkylmethacrylic acid and/or salts thereof    -   (15) Distillation column III for removal of high boilers    -   (16) Distillation column IV for removal of low boilers    -   (17) Distillation column V for final purification of MMA    -   (18) Distillation column VI for reducing the amount of alkyl        methacrylate in the bottoms stream from (15) (optional)    -   (19) Phase separator I for workup of the low-boiling fraction        from column IV (16)    -   (20) Distillation column VII for recovery of MAL from organic        phase from phase separator I (19) (optional)    -   (21) Reactor IV (ester cleaver) for recovery of alcohol from        ester byproducts (for example saturated esters such as alkyl        isobutyrate, alkyl propionate) (optional)    -   (22) Apparatus for mixing the reactants mixer I (optional)    -   (23) Water treatment (optional)    -   (24) Water feed III to (19) (optional)    -   (25) Acid feed IV to (21) (optional)    -   (26) Alkyl methacrylate product stream    -   (27) Recycling stream containing MAL    -   (A1)/(A2) Aqueous waste stream    -   (B1)/(B2) Organic waste stream

EXPERIMENTAL SECTION Example 1—with Recycling of W4 into the Extraction(9)

The reaction of methacrolein with methanol in the presence of anoxygen-containing gas in the liquid phase to afford methyl methacrylate(MMA) was carried out in reactor II. The reactor output from reactor IIhad the following composition: MEOH 43.1% by wt, MAL 8.8% by wt, MMA37.0% by wt, H2O 6.6% by wt, MAL acetal 360 ppm, remainder 4.4% by wt.

The output from reactor II (2) was directly supplied to the extraction(9). The extraction was performed in an extraction column. A partialwater feed II (11) to the extraction was carried out. The aqueous phaseP2 from the extraction was passed into column II (12). An aqueouswastewater stream A1 was discharged from the bottoms stream from thecolumn II. A partial recycling of the bottoms stream from the column II(W4) to the extraction (9) was carried out.

In the case of experiment 3 the addition of W4 and water was carried outat the top of the extraction column (9). In the case of experiment 4 theaddition of W4 was carried out below the top of the extraction columnand the addition of water was carried out at the top of the extractioncolumn (9).

The workup of the organic phase from the extraction was carried outusing the columns III (15), IV (16) and V (17). In the column V the MMAproduct stream (26) was withdrawn as the tops fraction.

The feeds into the extraction (9) and the amount of aqueous wastestreams are summarized in table 1 below.

TABLE 1 Summary of experiments 1-4 Extraction feeds Content of WaterWastewater Product methanol and feed II W4 stream A1 stream MMA MMA inW4 V no. kg/h kg/h kg/h kg/h % by wt pH W4 1 24.1 water / 29.2 15.65MEOH 2 (Ref) 0.50 acid W4 complete 500 ppm MMA 0 2 0 water 24.1 6.515.65 MEOH 2 0.19 acid (75% of W4) (25% of W4) 500 ppm MMA 0 3 8.8 water15.3 15.3 15.65 MEOH 2 0.30 acid (50% of W4) (50% of W4) 500 ppm MMA 0 48.8 water 15.3 15.3 15.65 MEOH 2 0.30 acid (50% of W4) (50% of W4) 500ppm (top) (below top) MMA 0

Example 2—with Recycling of W4 into Reactor III/Acetal Cleaver

The reaction of methacrolein with methanol in the presence of anoxygen-containing gas in the liquid phase to afford methyl methacrylate(MMA) was carried out in reactor II. The reactor output from reactor IIhad the following composition: MEOH 43.1% by wt, MAL 8.8% by wt, MMA37.0% by wt, H2O 6.6% by wt, MAL acetal 360 ppm, remainder 4.4% by wt.

The output from reactor II (2) was passed into the reactor III (8) forworkup. Reactor III was in the form of a continuously operated stirredtank with a downstream decanter. A partial feed of 96% sulfuric acid(acid S) to reactor III was carried out via acid feed I (10). The outputfrom reactor III was supplied to the extraction (9).

The extraction was performed in an extraction column. A partial waterfeed II (11) to the extraction was carried out. The aqueous phase P2from the extraction was passed into column II (12). An aqueouswastewater stream A1 was discharged from the bottoms stream from thecolumn II. A partial recycling of the bottoms stream from the column II(W4) to the reactor III and/or the extraction (9) was carried out.

The workup of the organic phase from the extraction was carried outusing the columns III (15), IV (16) and V (17). In the column V the MMAproduct stream (26) was withdrawn as the tops fraction.

The feeds into reactor III and the extraction are summarized in table 2below.

TABLE 2 Summary of experiments 5-7 Feeds Feeds Reactor III ExtractionContent of Feed I Feed II Wastewater MAL acetal Product methanol andwater & acid W4 water W4 A1 content in W2 stream MMA MMA in W4 V no.kg/h kg/h kg/h kg/h kg/h ppm kg/h % by wt pH W4 5 21.0 water / 3.1 /29.2 14 15.65 MEOH 2 (Ref) 0.50 acid (W4 complete) 500 ppm MMA 0 6 0Water 21.0 Water 3.1 / 9.6 14 15.65 MEOH 2 0.23 acid (69% of W4) (31% ofW4) 500 ppm MMA 0 7 0 Water 21.0 0 3.1 6.5 14 15.65 MEOH 2 0.19 acid(69% of W4) (10% of W4) (21% of W4) 500 ppm MMA 0

Example 3—with Recycling of W4 into Phase Separator I

The experiment was performed as described in example 1. The topsfraction from the column IV was admixed with water via the water feedIII (24) and subsequently separated in phase separator I(19) into anorganic phase P3 and into an aqueous phase P4. The aqueous phase P4 wasrecycled into the column II (12).

A partial recycling of the bottoms stream from the column II (W4) to thephase separator I and/or to the extraction was carried out. The feedsinto phase separator I and the extraction are summarized in table 3below.

TABLE 3 Summary of experiments 8-10 Feeds Feeds Phase separator IExtraction Content of Feed III Feed II Wastewater Product methanol andwater W4 water and acid W4 A1 stream MMA MMA in W4 V no. kg/h kg/h kg/hkg/h kg/h kg/h % by wt pH W4 8 2.5 / 24.1 water / 29.2 15.65 MEOH 2(Ref) 0.5 acid (W4 complete) 500 ppm MMA 0 9 0 2.5 24.1 water / 26.715.65 MEOH 2 (9% of W4) 0.5 acid (91% of W4) 500 ppm MMA 0 10  0 2.5 0water 24.1 4.0 15.65 MEOH 2 (8% of W4) 0.19 acid (78% of W4) (14% of W4)500 ppm MMA 0

Example 4—with Recycling of W4 into Reactor IV/Ester Cleaver

The experiment was performed as described in example 3. The phaseseparator I was provided with water via water feed III (24). The aqueousphase P4 from phase separator I (19) was passed into the reactor IV(ester cleaver) (21) and via acid feed IV mixed with a 96% sulfuric acid(acid S). The aqueous phase P5 from reactor IV was recycled into columnII (12).

A partial recycling of the bottoms stream from the column II (W4) to thereactor IV and/or to the extraction was carried out. The feeds into thereactor IV and the extraction are summarized in table 4 below.

TABLE 4 Summary of experiments 11-13 Feeds Feeds Reactor IV ExtractionContent of Feed IV Feed II Wastewater Product methanol and acid W4 waterand acid W4 A1 stream MMA MMA in W4 V no. kg/h kg/h kg/h kg/h kg/h kg/h% by wt pH W4 11 0.016 / 24.1 water / 29.2 15.65 MEOH 2 (Ref) 0.5 acid(W4 complete) 500 ppm MMA 0 12 0 1.4 24.1 water / 29.2 15.65 MEOH 2 (5%of W4) 0.5 acid (95% of W4) 500 ppm MMA 0 13 0 1.4 0 water 24.1 6.515.65 MEOH 2 (4% of W4) 0.19 acid (75% of W4) (21% of W4) 500 ppm MMA 0

It has been found that the process mode according to the inventionallows savings in water and/or acid feeds and minimization of theaqueous wastewater stream A1 at identical product quality and yield.

1-16. (canceled) 17: A process for producing alkyl methacrylates, theprocess comprising: producing methacrolein in a first reaction stage ina reactor I, and oxidatively esterifying the methacrolein with analcohol in a second reaction stage in a reactor II in the presence of anoxygen-containing gas, to form reaction water in a liquid phase and toafford an alkyl methacrylate, wherein a workup of a reaction mixturefrom the reactor II to afford the alkyl methacrylate comprises at leastone distillation and at least one extraction; wherein an aqueous phasecontaining the alcohol and an alkali metal and/or alkaline earth metalsalt of a Brønsted acid from the at least one extraction is treated inat least one second distillation in a column II to form, in a bottom ofthe column II, a process water stream containing the reaction water andthe alkali metal and/or the alkaline earth metal salt of a Brønstedacid, wherein in the process water stream a content of the alcohol andthe alkyl methacrylate is less than 5% by weight based on a totalprocess water stream, and wherein the process water stream from thebottom of the column II is partially discharged from the process andsent for disposal and partially recycled into the workup of the reactionmixture from the reactor II. 18: The process according to claim 17,wherein the process water stream from the bottom of the column II ispartially recycled into the at least one extraction, and is contactedwith an alkyl methacrylate- and alcohol-containing organic phase in theat least one extraction. 19: The process according to claim 17, whereinthe process water stream from the bottom of the column II is partiallyrecycled into a reactor III, and is contacted in the reactor III with analkyl methacrylate- and alcohol-containing organic phase which has acontent of methacrolein acetal of less than 3% by weight based on thealkyl methacrylate- and alcohol-containing organic phase. 20: Theprocess according to claim 17, wherein the at least one extraction isperformed in an extraction column and/or in a serially arranged seriesof at least two mixer-settler apparatuses, and wherein the process waterstream from the bottom of the column II is added below a top of theextraction column or in a mixer region of a mixer-settler apparatus ofthe at least two mixer-settler apparatuses, and water is optionallyadded in the top of the extraction column. 21: The process according toclaim 20, wherein the addition of water and optionally, of a Brønstedacid, to the at least one extraction is carried out above the additionof the process water stream from the bottom of the column II. 22: Theprocess according to claim 19, wherein an addition of a Brønsted acid iscarried out in the reactor III and the addition is chosen such that incontinuous operation a pH in the range from 1.5 to 2.5 is established inthe process water stream from the bottom of the column II. 23: Theprocess according to claim 22, wherein the addition of the Brønsted acidis carried out exclusively via the addition in reactor III. 24: Theprocess according to claim 17, wherein the at least one extractionaffords an organic phase containing alkyl methacrylate and methacrylicacid and the organic phase from the at least one extraction is separatedin a distillation stage IV into a bottoms fraction containing alkylmethacrylate and a lower-boiling tops fraction. 25: The processaccording to claim 24, wherein the tops fraction from the distillationstage IV is admixed with water and subsequently separated in a phaseseparator I into a further organic phase and into a further aqueousphase. 26: The process according to claim 25, wherein the furtheraqueous phase from the phase separator I is mixed with at least oneBrønsted acid in a reactor IV, wherein ester byproducts present in thefurther aqueous phase from the phase separator I are cleaved and alcoholis recovered, and the process water stream from the bottom of the columnII is optionally partially passed into the reactor IV. 27: The processaccording to claim 26, wherein a product from the reactor IV iscompletely or partially passed into the column II. 28: The processaccording to claim 25, wherein the further aqueous phase from the phaseseparator I is completely or partially passed into the column II. 29:The process according to claim 17, wherein the reaction mixture from thereactor II is separated in a distillation stage I, wherein methacroleinand in part alcohol are removed via a tops fraction and recycled to thereactor II. 30: The process according to claim 29, wherein themethacrolein is completely or partially added to the distillation stageI and passed to the reactor II via the tops fraction from thedistillation stage I. 31: The process according to claim 17, wherein thealcohol is methanol and the alkyl methacrylate is methyl methacrylate.32: The process according to claim 22, wherein the Brønsted acid issulfuric acid. 33: The process according to claim 20, wherein the wateroptionally added in the top of the extraction column is demineralizedwater. 34: The process according to claim 17, wherein the Brønsted acidis used. 35: The process according to claim 17, wherein the Brønstedacid is not used. 36: The process according to claim 17, wherein theworkup of the reaction mixture further comprise a reactor III, and wherethe process water stream from the bottom of the column II is partiallyrecycled into the reactor III.